1. Field of the Invention
The present invention relates to a communication apparatus. In particular, the present invention relates to a communication apparatus capable of selecting either an ASK (or BPSK) modulation scheme or a non-ASK (or non-BPSK) modulation scheme and transmitting a modulated signal.
2. Description of the Background Art
Generally, in the case where communication is performed using a modulated signal, the modulation scheme may be selected according to the condition of a transmission path or the amount of data to be transmitted. For example, in a DSRC (Dedicated Short-Range Communication) system used in Intelligent Transport Systems, ASK and QPSK modulation schemes can be used.
FIG. 12 is a diagram showing an exemplary conventional communication apparatus capable of selecting a modulation scheme. In the conventional communication apparatus, in either case of ASK or QPSK modulation, a modulated high-frequency signal is obtained by multiplying a band-limited baseband signal by a high-frequency signal (see, for example, US Patent Publication No. US2003/0157888A1 (FIG. 5); the corresponding parent application is Japanese Laid-Open Patent Publication No. 2003-244260).
In FIG. 12, the conventional communication apparatus includes an ASK data generation section 201, a QPSK data generation section 202, a data selection section 203, low-pass filter sections 204 and 205, a quadrature modulation section 206, a signal source 207, a high-frequency power amplification section 208, and a transmission antenna 209. Note that although there is no specific description of the data selection section 203 in the drawing of the aforementioned patent publication, the data selection section 203 is additionally described because it is essential to provide an element which prevents collisions between the output of the ASK data generation section 201 and the output of the QPSK data generation section 202.
The ASK data generation section 201 generates a piece of ASK data from predetermined data. The QPSK data generation section 202 generates pieces of QPSK data in a pair from predetermined data. The data selection section 203 is a switch which has two input terminals each having two terminals in a pair and one output terminal having two terminals in a pair, and which controls the connection switching between the input terminals and the output terminal based on a modulation scheme designation signal. In the example of FIG. 12, the piece of data outputted from the ASK data generation section 201 and a piece of data having a value of 0 are inputted as a pair to one of the input terminals (input 1), and the pieces of data in a pair outputted from the QPSK data generation section 202 are inputted to the other input terminal (input 2). The low-pass filter sections 204 and 205 remove high-frequency components from pieces of data in a pair outputted from the data selection section 203, respectively, and output the resulting pieces of data as baseband modulation signals. The quadrature modulation section 206 performs a frequency conversion on the baseband modulation signals outputted from the low-pass filter sections 204 and 205 using a signal generated by the signal source 207, and then outputs a high-frequency signal modulated using ASK or QPSK. The high-frequency power amplification section 208 amplifies the high-frequency signal modulated by the quadrature modulation section 206 and radiates the resulting signal from the transmission antenna 209.
In the case of performing ASK modulation, a modulation scheme designation signal designating an ASK modulation scheme is provided to the data selection section 203. In response to this, the data selection section 203 switches the connection to the input 1 and the output terminal, and outputs a single-axis baseband modulation signal outputted from the ASK data generation section 201 and a signal having a value of 0, to the low-pass filter sections 204 and 205.
In the case of performing QPSK modulation, a modulation scheme designation signal designating a QPSK modulation scheme is provided to the data selection section 203. In response to this, the data selection section 203 switches the connection to the input 2 and the output terminal, and outputs baseband modulation signals in a pair outputted from the QPSK data generation section 202, to the low-pass filter sections 204 and 205.
Now, the relationship between the selecting of modulation schemes (ASK/QPSK) and the transmission power outputted from the high-frequency power amplification section 208 will be described. The transmission power is determined by the power of a high-frequency signal outputted from the quadrature modulation section 206 and the gain of the high-frequency power amplification section 208. The power of the high-frequency signal outputted from the quadrature modulation section 206 is determined, in the case of performing ASK modulation, by the amplitude of data outputted from the ASK data generation section 201 and the coefficients of the low-pass filter sections 204 and 205, and determined, in the case of performing QPSK modulation, by the amplitude of data outputted from the QPSK data generation section 202 and the coefficients of the low-pass filter sections 204 and 205. Therefore, the ratio of the transmission power outputted from the high-frequency power amplification section 208 when performing ASK modulation to the transmission power outputted from the high-frequency power amplification section 208 when performing QPSK modulation has a fixed value which is determined by the design.
A communication system is normally composed of a plurality of communication apparatuses. The communication apparatuses have different factors of importance depending on the type thereof. For example, in the case where the communication apparatus is configured as a base station for a wireless communication system, since the communication apparatus exerts a great influence on the operation of the entire system, importance is placed on reducing transmission errors. In the case where the communication apparatus is configured as a mobile terminal, since the communication apparatus is owned by general users, importance is placed on reducing costs.
Of the elements composing the communication apparatus, a high-frequency power amplification section is one of those elements that place importance on both reducing transmission errors and reducing costs. In the communication apparatus placing importance on reducing transmission errors, it is desirable to select a high-frequency power amplification section capable of outputting a high power. On the other hand, in the communication apparatus placing importance on reducing costs, it is desirable to select a low-cost high-frequency power amplification section even if its maximum possible output power is somewhat low. In selecting a high-frequency power amplification section, it should be noted that the maximum possible output power varies with the modulation scheme.
The high-frequency power amplification section will be described using a DSRC system as an example. According to the standard for the DRSC system, it is specified that in the base station and mobile terminal of class 1, the upper limit of transmission power is 10 mW in both cases of ASK modulation and π/4 shift QPSK (hereinafter referred to as “QPSK”) modulation. The transmission power is defined as the peak value of the antenna power in the case of ASK modulation, and is defined as the average value of the antenna power in the case of QPSK modulation. However, in a low-cost high-frequency power amplification section designed to output a peak value of 10 mW when performing ASK modulation, such an amplification section can only output an average value of the order of 5 mW when performing QPSK modulation.
In the communication apparatus placing importance on reducing transmission errors, since there is a constraint that the sensitivity of a cost-conscious communication apparatus with which it communicates is not so good, it is desirable to select a high-frequency power amplification section capable of outputting a power of 10 mW when performing QPSK modulation. Therefore, it is desirable that the transmission power in the case of performing ASK/QPSK modulation be 10 mW/10 mW which is the upper limit specified in the standard. On the other hand, in the communication apparatus placing importance on reducing costs, it is desirable to select a low-cost high-frequency power amplification section, though the output power in the case of performing QPSK modulation is on the order of 5 mW. Therefore, it is desirable that the transmission power in the case of performing ASK/QPSK modulation be 10 mW/5 mW.
Namely, the ratio of the transmission power outputted from the high-frequency power amplification section when performing ASK modulation to the transmission power outputted from the high-frequency power amplification section when performing QPSK modulation varies with the type of communication apparatus.
In the conventional communication apparatus, however, the ratio of the transmission power outputted from the high-frequency power amplification section when performing ASK modulation to the transmission power outputted from the high-frequency power amplification section when performing QPSK modulation has a fixed value which is determined by the design of the data generation sections and the low-pass filter sections. Accordingly, there is a need to provide plural types of data generation sections or low-pass filter sections depending on the type of communication apparatus, which makes it impossible to sufficiently enjoy cost-reduction advantages resulting from mass production.
In the case where data generation sections and low-pass filter sections which are not suitable for the type of a given communication apparatus are used, there is a need to provide an expensive high-frequency power amplification section. For example, the case is described where a communication apparatus placing importance on reducing costs uses data generation sections and low-pass filter sections which are designed such that the transmission power ratio when performing ASK/QPSK modulation is “1”. In this case, if a low-cost high-frequency power amplification section is used, the transmission power is 5 mW not only when performing QPSK modulation but also when performing ASK modulation, causing a problem in transmission reliability. That is, a low-cost high-frequency power amplification section cannot be used.
In order that one communication apparatus realizes two different transmission power ratios in ASK/QPSK modulation, i.e., 10 mW/10 mW=1 and 10 mW/5 mW=2, normally, the communication apparatus requires the following configuration (A) or (B).
(A) A multiplier for amplifying amplitude by a factor of √{square root over (2)} is provided at any point before the quadrature modulation section 206 of FIG. 12. In this configuration, there is a need to handle the number “about 1.4 times” with which, when numbers are represented in binary format, the number of binary digits gets large, and as a result, the size of hardware for digital signal processing increases, causing an increase in costs.
(B) A variable attenuator or a gain-variable amplifier is provided to the input of the high-frequency power amplification section 208 of FIG. 12. In this configuration, the addition of a high-frequency element degrades the stability of transmission power and increases costs.